Automatic volume control system



Feb 1938 F. WILLGING 2,107,514

AUTOMATIC VOLUME CONTROL SYSTEM Filed May 24, 1930 MRS Outpu t.

Def. Grid Current-Watt Output Input 7 1NI /ENTOR.

ATTORNEYS Patented F eb. 8, 1938 UNITED STATES PATENT OFFICE AUTOMATIC VOLUME CONTROL SYSTEM Application May 24, 1930, Serial No. 455,353 I 1 Claim.

My invention is addressed to automatic volume control systems for radio sets as will hereinafter be explained, and is particularly directed to an improvement upon systems in which volume or sensitivity control is attained through a variation in the grid bias of one or more tubes in the radio frequency end of the receiver. A primary object of my invention is the provision of means to effect volume control without diminishing the in maximum sensitivity of the set. That is to say, it is my object to efiect volume control by means which do not become active until the point of overload is substantially reached. It is an object of my invention to control volume automatically 15 without the provision of an extra tube or the like, and with a minimum of expense and additional parts. t is an object of my invention to control volume quite independently of tube variations.

It is an additional object of my invention to 20 eliminate in a radio set that distortion which is due to detector tube overload.

These and other objects of my invention which will be apparent to one skilled in the art upon reading these specifications, I accomplish by that certain construction and arrangement of parts of which I shall describe a preferred embodiment, reference being made to the drawing which forms a part of these specifications.

In the drawing:-

Figure 1 is a graph illustrative of the principle upon which my invention operates.

Figure 2 is a diagrammatic showing of a portion of a radio set embodying my invention.

It has been shown that the detector tube is in reality the bottle neck of a radio set. As the input upon the tube is increased the output in watts increases to a certain maximum point at which the tube begins to overload, whereupon although the input is further increased, the output begins .14 to decline. Furthermore with modern tubes the point at which distortion begins to be apparent lies not very far beyond the point at which overload begins, so that irrespective of the volume, distortion begins to appear in the set when the detector tube has been overloaded.

It has been proposed to control volume automatically by varying the grid bias of one or more tubes in the radio frequency end of the set, the variation being accomplished through variations 5-1) in the plate voltage of a succeeding tube, usually a detector. For the most part this has been accomplished indirectly, requiring the use of an additional tube or tubes which do not otherwise form a functioning part of the radio set in the 5 sense that they are parts neither of the amplifying system nor of the detector systems. One difficulty inherent in these systems is that in most prior proposals the sensitivity control begins at once, so that the overall efiiciency of the set is greatly diminished. In a co-pending application, Ser. No. 402,771 filed Oct. 26, 1929 in the name of Emmery J. H. Bussard, Patent No. 1,978,008 of October 23, 1934, an improvement upon such systems is set forth which does not require the use of any additional tube in the set. In another co-pending application Ser. No. 392,463, filed Sept. 13, 1929, in the name of Dorman D. Israel, Patent No. 2,012,392 of August 27, 1935, an amplifying system is set forth so balanced that it does not begin to function until a predetermined level has been reached.

In my invention, while I gain the named advantages of the systems referred to as being set forth in the co-pending applications, namely, that I eliminate the use of any extra tubes and achieve a volume control which does not become effective below a predetermined level, I depart from all prior systems of which I am aware in this, that I do not effect volume control through variations in the plate current of any tube in the set. I effect volume control through varying the grid bias of one or more of the radio frequency tubes; but I employ a new source of the potential used in doing this, which source does not become operative so long as the tube in which it origi- 3O nates is not overloaded.

In Figure 1, I have plotted the watt output and grid current, as shown by curve A and B, of a tube such as the detector tube, the watt output and grid current of the detector tube being shown vertically and the input in micro-volts per meter being shown horizontally. It will be seen that as the input is increased the output increases sharply to a certain maximum at which the tube begins to overload, whereupon, as the input is increased the output diminishes and distortion begins to become apparent.

At or just before the point at which the tube begins to overload, i. e., at about the point of maximum output of the tube, the grid begins to become so electro-positive with respect to the cathode that the grid itself draws current. This is shown in curve B in Figure 1, and it will be noticed that the rise in this curve is very rapid, indicating that as the input is increased beyond the overload point the current drawn by the grid increases quite steeply. I make use of this grid current to effect volume control by coupling it back to the grids of a preceding tube or tubes so as to affect the bias thereof. I have found that the point of distortion in all of the sets in which I have embodied my invention does not lie forwardly in the curve of Figure 1 of that point at whichthe tube begins to overload and the current drawn by the grid begins to control the volume; and I have found further that the increase in the grid current is so sharp as to effect substantially a perfect control of the volume throughout the broadest variations of input commercially to be met with. In other words, in a properly designed set embodying my invention the curve A in Figure 1 will be as shown up to the maximum point, at which instead of coming down again, it will flatten out and follow such a path that the output of the set remains substantially constant.

In Figure 2 I have shown my invention diagrammatically. All of those parts of the circuit not necessary to an understanding of my invention being eliminated. I have shown my system to be operated from the grid voltage of a detector tube, although it should be understood that it can be applied to other tubes in the set, including the radio frequency tubes and the audio frequency tubes. I find it more advantageous to apply it to the detector tube because, as hereinabove indicated, no set is capable of reception without distortion substantially beyond the point of overload of the detector tube. I have shown a radio frequency tube 2 and a detector tube I, both being of the four element or screen grid type, but it will be understood that my invention is not restricted to this type of tube. The electron emitting element of this tube I have desig nated at 3, the control grid at 4, and the plate at 5, the grid being in series with the usual inductance 6. The filament connections and connections to the other electron emitting device in the tube are not important. When the grid 4 at or about the point of overload of the detector tube begins to draw current, a resistance I placed in series with the grid will have developed across it a voltage dependent in amount, of course, upon the amount of current drawn by the grid and magnitude of resistance 1. I make use of this Voltage to affect the grid bias of one or more tubes 2, preferably in the radio frequency stage, and occurring ahead of the tube 5. I do this by connecting one end of the resistance 1, by means of a conductor 8, preferably through an isolating resistance or filter 9, the purpose of which will be readily understood, to the grid of the tube or tubes 2 as shown. The other end of the resistance l is of course connected with the ground lead l I, to which the electron emitting device I2 of the tube or tubes 2 is also connected through an appropriate resistance 53, bypassed by a condenser if desired.

It will be understood that the connections may be modified as found desirable in different types of sets. What I do essentially is to control the volume through affecting the sensitivity of a tube or tubes in the set by modifying the grid bias thereof, the means whereby the modification is made effective being the grid current drawn by a succeeding tube, at and above the point at which that tube begins to overload. While I prefer to effect my control of volume from the grid voltage of the detector tube for the reasons indicated, it will be readily understood that the same system may be made to work from an audio tube. I apply the voltage drawn by or developed in the grid of a tube such as the detector to preceding tubes between the control grid and the cathode. As the controlling tube begins to overload I thus bring about an increase in the bias on the controlled tube or tubes which in turn decreases their sensitivity and amplification. Consequently the input to the controlling tube is reduced to a point where it will balance with the eifect of the grid current; and I have found in the sets to which I have applied my invention I that this balance serves to keep the set operating cut down the overall efiiciency of the set because the volume or sensitivity control does not become effective until at or about the point of maximum voiume without distortion. Consequently although I provide no disconnection means, my system of amplification control does not diminish the voliune of signals below the maximum volume which the tubes are capable of handling. In other words, my control means does not become effective until a predetermined level is reached, and it has a further advantage in that the level is determined by the characteristics primarily of the controlling tube, so that any tubes which I use for the purpose are capable of operating at their maximum efficiency.

Again, I neither use in my set any extra tubes or the like, nor any expensive additional apparatus. It will be understood that although the connections as illustrated diagrammatically in Figure 2 may be varied in different sets without departing from the spirit of my invention, yet in general they will comprise merely a resistance across which a potential may be builtup when the grid of the controlling tube draws current, and means for impressing this potential upon the grid of a preceding tube or tubes, which employs merely a connection and an isolating resistance where such resistance is deemed necessary.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is:--

In radio apparatus, a detector tube and a controlled tube, the grid of said detector tube being connected to ground through a resistance, the

ungrounded end of said resistance being connected through a resistance to the grid circuit of said controlled tube and also to a point of poten tial more negative than the electron emitting device of said detector tube and said controlled tube, thus establishing an initial bias on these tubes, the cathode of said detector tube being connected to ground through a resistance.

LOUIS F. WILLGIN G.

A ma 

